Well Oldmogman, I've a double garage that's 22ft wide and 23ft deep and it's still too small! Go for the biggest you can, even a few extra inches is worth it. You'll find that "Her Indoors" will demand that the freezers, tumble dryer etc, will go in there so every inch you gain is important.
Regarding insulation, I live in a North Hampshire frost pocket and on clear calm winter nights we can go down to -8C/-10C in most winters and occasionally -13C. Inside the uninsulated garage it'll get down to only -3C even though the walls are only one brick width thick (i.e. 4 inches) between piers. I can live with this, but when a warm front blows in the sudden temperature rise is always accompanied by relatively high humidity so then I keep the doors shut and allow the internal temp to rise to the outside using very little ventilation. Insulation would be preferable but isn't an easy retrofit. But on most days there's no problem and the doors can be left open without worries. I have a cheap oscillating fan heater to blow underneath the car after wet journeys that helps things.
Regarding a pit - do it! I wish I had one but again retrofit with pipes and sewers underneath it's not on. I only see underneath in comfort during MOTs (they allow me in the pit) and I wish I could do it more often.

Other thoughts. Before you move in paint the internal walls white - an old roller and old cans half empty are the thing. It'll improve the lighting more than you think. And board the roof (I've used these https://www.wickes.co.uk/Wickes-Chipboa ... 3/p/116420 ) again painting the underside white. Regarding lights, put more in than you think you need - you've never got too much light! I've used these IKEA lights a lot https://www.ikea.com/gb/en/products/lig ... -60262659/ with 5 watt GU10 LED bulbs. (I take out the plastic lens on them for broader beams.) Being able to aim them is useful. And have lots of sockets! I've had to retrofit mine with triples!
Regards the doors, the side opening variety have the advantage that you can open them and then throw a sheet over the top to give you a dry "tunnel" to back out into; it has its uses. Re windows, I found when mine was bricked up as part of an extension the added shelf space was more use that the previous light for I always needed the light on for serious work. Finally, I've a tap and sink in the corner which is far more use than I ever thought it'd be.
Regards from MikeN.

Morris Minor, the car of the future. One day they will all look like this!

One thing I forgot is that over my two double side hung doors are 9 inch by 5 inch "Catnic" lightweight "I" beams on which I can hang my hoist to lift out the engine and gearbox assembly. Not something you can do with "up and over" systems.
Also I thoroughly recommend you that you don't have a bottom sill on your side door. Your builders will fit an ordinary door frame if you don't watch it and you'll trip over it when you're carrying something heavy. I removed mine about 5 years ago - should have done it 20 years ago!
MikeN.

Morris Minor, the car of the future. One day they will all look like this!

I spent all my spare time last year adding an extension on my garage, It was done with concrete block laid flat as the rest of the building is solid brick then we added some insulation and timber cladding, it looks better now and is a good workspace.
This is how we started